1. Field of the Invention
The present invention relates to an electrode assembly for an electrostatic lens, and more particularly to an electrode assembly for an electrostatic lens which can be used in a color cathode ray tube having three in-line arranged electron guns.
2. Description of Related Art
One important factor which restricts resolution of cathode ray tubes is a spherical aberration of an electrostatic lens in cooperated in the cathode ray tube. For high resolution, it is required to enlarge an aperture of an electrode for an electrostatic lens so that the spherical aberration becomes small. However, in a color cathode ray tube having three in-line arranged electron guns associated with in-line arranged electrostatic lens, if the aperture diameter of the electrostatic lenses is simply increased, a "separation distance" between the centers of the apertures of the in-line arranged lenses must be increased. This will require an enlarged diameter of evacuated envelope neck accommodating the electron gun therein.
In general, increase of the separation distance will decrease convergence error in that three electron beams can be intersected on any point of a phosphor screen. Further, increase of the envelope neck requires a large deflection power. In order to enlarge the aperture of a main electrostatic lens without changing the separation distance of the main electrostatic lens and the envelope neck diameter in the in-line electron gun type cathode ray tube, the applicant has proposed in Japanese Patent Application Laid-open No. Sho 60-9036 an electrode for an electrostatic lens, which comprises a tubular portion closed at its one end and having the configuration obtained by laterally combining three in-line arranged circular cylinders in partially overlapped manner, the cylinders having an inner diameter larger than a predetermined separation distance, and three independent circular collars outwardly projecting from a bottom of the tubular portion.
Referring to FIGS. 1 to 3, there is shown the electrode proposed in the above Laid-open Japanese Patent Application for use in an electrode assembly for an electrostatic lens. FIG. 1 is a partially broken perspective view of the electrode disclosed in the above Japanese Patent Application, and FIGS. 2 and 3 are top view and sectional view of the tubular electrode shown in FIG. 1, respectively.
The shown electrode 10 includes a tube-shaped casing 12 of a substantially rectangular section having a pair of straight parallel long sides and a pair of arced short sides. The casing 12 has a flange 14 provided at its one end to extend outwardly. The flange 14 has a pair of support tags 16 each integrally formed at a center portion of the long side.
The casing 12 is closed at its other end by a first end wall 18, from which a tubular portion 20 projects towards the inside of the casing 12. The tubular portion 20 has a configuration obtained by laterally combining three in-line arranged circular cylinders 20R, 20G and 20B in a partially overlapped manner. Each of the cylinders 20R, 20G and 20B has an inner diameter D.sub.0 larger than a given separation distance S. Further, the tubular portion 20 has a narrow portion 26 formed at a junction between each pair of adjacent cylinders 20R-20G and 20G-20B. Thus, the tubular portion 20 forms a concave space on the end wall 18.
The tubular portion 20 terminates at a second end wall 22 separate from the first end wall 18 by the distance h.sub.0. The second end wall 32 has three independent collars 24R, 24G and 24B projecting therefrom toward the inside of the casing 12. These rollers 24R, 24G and 24B are coaxial with the cylinders 20R, 20G and 20B of the tubular portion 20, respectively, and have an inner diameter D.sub.1 smaller than that of the cylinders 20R, 20G and 20B. The collars 24R, 24G and 24B has the section of a perfect circle and a height of h.sub.1.
Turning to FIG. 4, there is shown in a sectional view an in-line type electron gun for a color cathode ray tube associated with a bi-potential type focusing elecrostatic lens which comprises a pair of tubular electrodes having the structure as mentioned above as a third grid electrode and a fourth grid electrode, the pair of tubular electrodes being located with the end walls 18 of the tubular electrodes facing each other.
The shown electron gun includes three cathodes 30R, 30G and 30B arranged in the in-line manner and containing a filament 30 therein, respectively. In front of the cathodes 30R, 30G and 30B, a first grid electrode G.sub.1 and a second grid electrode G.sub.2 are located in the named order, separately from the cathodes and from each other. These grid electrodes have three openings for passage of three electron beams emanated from the three cathodes 30R, 30G and 30B.
Further, in front of the second grid electrode G.sub.2 there are located a third grid electrode G.sub.3 and a fourth grid electrode G.sub.4. The third grid electrode G.sub.3 includes a first tubular electrode portion 34 having an end wall close to the second grid electrode G.sub.2 and a second tubular electrode portion 36 having the structure shown in FIGS. 1 to 3. The first electrode portion 34 is bonded at its open end 34A to the flange 14 of the second tubular electrode portion 36.
With the above arrangement, the electron beam passage holes of the first to third grid electrodes G.sub.1, G.sub.2 and G.sub.3 are coaxial to each other and are separated from each other by the separation distance S in each of the electrodes. But, the electron beam passage holes of the fourth grid electrode G.sub.4 are separated from each other by a separation distance S.sub.1 which is slightly larger than the separation distance S of the third grid electrode G.sub.3. In addition, the center electron beam passage hole of the fourth grid electrode G.sub.4 is coaxial to that of the third grid electrode G.sub.3.
In the arrangement as mentioned above, the tubular electrodes 36 and 38 constitute a bi-potential type focusing electrostatic lens which has a very small spherical aberration by means of interaction between the tubular portion 20 composed of three combined large-diameter cylinders and the three independent collars 24R, 24G and 24B projecting from the tubular portion 20. In addition, the action of the lens in an arrangement direction of the three electrostatic lenses, i.e., in a horizontal direction is substantially equal to that of the lens in a direction perpendicular to the horizontal direction, so that the electrostatic lenses can have a very high resolution over a wide electron beam current range including a heavy electron beam current which will enlarge the diameter of the electron beam in the aperture of the electrostatic lens.
As mentioned above, the electrode 10 for the electrostatic lens has such a complicated structure that the three independent collars 24R, 24G and 24B are formed integrally of the bottom of the tubular portion 20 having the configuration obtained by laterally combining the three in-line arranged circular cylinders 20R, 20G and 20B in the partially overlapped manner. Therefore, it is very difficult to manufacture the electrode 10. Specifically, it is difficult to form the tubular portion 20 having the configuration of the three laterally overlapped cylinders and to form the three independent collars 24R, 24G and 24B of the height h.sub.1 projected from the bottom of the tubular portion 20. Particularly, if the ratio of the height h.sub.1 to the diameter D.sub.1 of the independent collars becomes large, or if a high axial symmetry is required in each of the independent collars, difficulty in working the electrode will be increased still more.
Furthermore, if the inner diameter D.sub.0 of the cylinders 20R, 20G and 20B defined in the tubular portion 20 is increased, the spherical aberration of the three in-line electrostatic lenses can be decreased. But, if the overlap degree of the increased diameter cylinders 20R, 20G and 20B is increased so as to prevent the increase of the separation distance, the inner distance G in the narrow portion 26 formed at the junction between each pair of adjacent cylinders 20R-20G and 20G-20B is increased. As a result, a center electrostatic lens of the three electrostatic lenses will have a large astigmatism in which the focusing action in a horizontal direction is smaller than that in a vertical direction. Accordingly, an optimum voltage for focus in the horizontal direction is required to be lower than that for focus in the vertical direction. Thus, the electron beam passing through the center electrostatic lens forms an oblong circle spot having a horizontal long axis on the phosphor screen. In other words, the resolution given by the center electrostatic lens becomes lower than that given by the side electrostatic lens.